Outside Golden Gate Park in San Francisco last Saturday a Californian was touting for business. "Hey mister, want tickets for the Fleed?" Inside the park there were 30,000 people enjoying a day-long festival of Irish music and culture headlined by Van Morrison and Elvis Costello - and almost as many opinions about how to pronounce the word Fleadh.

The Anglo-Irish dictionary suggests you say "flar" to rhyme with car, and that it means a festival of music. The word has its origins in county fairs and traditional music gatherings such as the huge annual session at Lisdoonvarna in Co Clare - but these days it also represents a transatlantic franchise.

Yesterday there was a Fleadh event in Chicago; on Saturday it will move to Boston, and the following week New York. A core group of like-minded performers is to play at every event with support from local acts, and Elvis and Van will return to headline the tenth London Fleadh on 10 July.

The man behind it all is Vince Power, organiser of the Leeds, Reading and Homelands Festivals, and owner of 15 venues. A decade ago most people thought he was mad to plan a huge Irish festival in Finsbury Park in north London: the police were scared of a riot, the council worried about drunks, and the music press wondered how he would find enough decent acts to fill a whole day.

The former furniture salesman from Co Waterford booked 26 bands, most of which had already appeared at his first venue, the Mean Fiddler in Harlesden, north London. Fleadh 90 was just in time to catch the start of a huge cultural wave which Power has been surfing ever since. "It was practically cool to be Irish in Britain, something that hadn't always been the case," remembers the Northern Irish writer and broadcaster Stuart Bailie. Van Morrison was back in the charts, and a new generation of performers had been inspired to link traditional music with rock and roll. Meanwhile, Irish comedians such as Sean Hughes were making it on the comedy circuits; the breweries were planning to open Irish theme pubs across Europe; and a new band of emigrants had swelled the population of first- and second-generation Irish in Britain to two million.

"Many of the new arrivals were young and highly educated, in search of prestigious jobs," says Bailie. "On that day in 1990 it felt like most of them descended on Finsbury Park."

There were actually only 30,000 people but it was sold out. Then as now it was a family festival, with stalls run by Irish community groups. Fleadh 90 raised pounds 20,000 for the Migrant Training Scheme to help young employed people; last year it enabled the Dublin charity Cradle to rebuild a school in Mostar.

Brian Kennedy sang at the first event and thought it extraordinary. "Don't forget that 25 years ago there were still signs up outside lodging houses in London saying 'No Irish No Blacks No Dogs', and yet the first Fleadh was the sign that things were changing for us." Like many others Kennedy had moved from Belfast to London as a teenager, with the address of a squat and pounds 15. After five years touring with Van Morrison he has a new solo deal with Epic in America. "If someone said 'Irish music' to you back then, a scratchy record would probably have started up in your head going 'A-diddly, diddly, diddly-dee'. Now people know about Riverdance, but they also know about Sinead and Ash - the greatest thing about Irish music is its diversity."

The Fleadh became too diverse for some people. Vince Power admits that after a few years the supply of new headline acts dried up. Talent developed too slowly, or else went straight from Fleadh support-slot to stadium. Ireland's booming economy slowed the flow of emigrants. There was no option but to broaden the festival's appeal into something more widely Celtic. Scots such as Runrig were drafted in, but sometimes the idea seemed to be to invite anyone who had ever seen a pint of stout - including Sting, The Beautiful South, Suzanne Vega, and this year The Pretenders. The charitable put it down to the inclusive nature of Irish music. Others saw it differently.

"I remember playing the Fleadh with the Maori Choir and the Pacific Island Log Drummers," says the New Zealander Neil Finn, who headlined in 1994 with his old band Crowded House. "The atmosphere was magic as the sun set. Shane MacGowan was at the side of the stage saying to the guy next to him, 'What are Crowded House doing on the bill? They're not f***ing Irish'."

MacGowan has played every single Fleadh. He is on the bill again this year, along with identifiably Irish acts such as the Saw Doctors, Afro- Celt Sound System and Altan, and the more unlikely Lonnie Donegan and John Martyn. Novelist and Father Ted star Ardal O'Hanlon will compere the comedy stage, which feature Perrier winner Tommy Tiernan.

Whoever plays, many people just go for that illusive Irish cliche the craic. Mark Hamilton of Ulster band Ash, who made their first London appearance at the festival in 1994, remembers watching Shane MacGowan - but his words could equally describe the behaviour of many merry members of the audience down the years. "He was totally hammered and singing out of time, and out of tune. It was very entertaining."

GET DOWN AND DIRTY HERE

GLASTONBURY FESTIVAL OF CONTEMPORARY PERFORMING ARTS

At 29, the grand-daddy of festivals is famous for its mud and sheer scale - three days of it which this year include Hole, Blondie, REM, Texas, The Corrs, Fatboy Slim and the Chemical Brothers.

Dates: 25 to 27 June

Tickets: pounds 85

Information: 0906 708 0808; Credit card hotline 0115 912 9129

THE FLEADH

Began nine years ago as a celebration of Irish music. Now brings international stars to Finsbury Park, north London. This year sees Van Morrison, Shane MacGowan and The Saw Doctors, plus Fleadh debuts from The Pretenders and Elvis Costello.

The best organised event of the summer: twin festivals in Hylands Park, Chelmsford and Weston Park, Staffordshire. Headliners include the Manics, Suede, Massive Attack and James Brown. The Happy Mondays, Supergrass, The Cardigans, plus Super Furry Animals, Orbital and Sly and Robbie are among the support.

Sixties Jazz and Blues festival that has emerged from a hard-rock rut. This year the ongoing re-birth of Reading includes a sister festival in Leeds. Expect performances from Catatonia, The Charlatans, Red Hot Chilli Peppers and Elastica. Plus Terrorvision, Beth Orton, Reef and The Divine Comedy.

Dance-meltdown and multi-media collaboration in the Larmer Tree Gardens in Wiltshire. Festival-goers can take refuge in the tea rooms, before exploring a line-up including Squarepusher, Mixmaster Morris and Coldcut's Matt Black.

Dates: 6 to 8 August

Tickets: Adults pounds 55, Children pounds 15

Information: 0181 372 9735

THE LIZARD

Inaugural festival, at Goonhilly Downs on the Lizard Peninsula, to coincide with the eclipse. More a holiday, with five days of bands and a club night on 10 August. Artists include Kula Shaker, Sasha, Urban Species, Femi Kuti, Bjorn Again and the Drummers of Burundi.